Braided thermionic cathode having emissive material



May 10, 1966 B. ANTONIS ETAL BRAIDED THERMIONIC CATHODE HAVING EMISSIVE MATERIAL Filed Feb. 7, 1962 BASIL ANTONIS LEONARD CHARLES WILTSHIRE lNVENTORS W WWW,

ATTORNEY United States Patent 3,250,943 BRAIDED THERMIONIC CATHODE HAVING EMISSIVE MATERIAL Basil Antonis and Leonard Charles Wiltshire, London,

England, assignors 'to Thorn Electrical Industries Limited, London, England Filed Feb. 7, 1962, Ser. No. 171,658 Claims priority, application Great Britain, Feb. 10, 1961,

5,006/ 61 8 Claims. (Cl. 313346) The present invention relates to thermionic cathodes for discharge lamps. While discharge lamps using such cathodes include low and high pressure mercuryvapour lamps and sodium vapour lamps the invention is of particular value in low pressure mercury discharge tubes with a fluorescent coating.

In such lamps it is known to use cathodes consisting of either a single coil or a coiled coil of tungsten wire with a thermionically emissive material such as a mixture of barium, strontium, and calcium oxides coated on the coil or held within the coil.

Discharge lamps including coils of this type may be started without cathode heating by the application of a high voltage or with cathode heating either by starter switches or by means of transformers. The cathodes according to the invention are applicable in all cases but are particularly advantageous in the case when starting is done by the use of cathode heating transformers.

The main requirement for such cathodes is that they should retain a high proportion of thermionic emitter without, at that same time, increasing the total weight of the cathode structure so that its thermal inertia is raised to such a degree that the starting time of the lamp is unduly prolonged. The importance of the amount of thermionic emitter in the cathode arises from the fact that it is used up during the life of the lamp and the quantity which is present in the cathode structure will be an important feature in determining the length of useful life of the lamp.

One of the objects of the invention 'is to provide such cathodes with a high ratio of thermionic emitter to wire weight, a condition necessary to ensure rapid starting of lamps employing such cathodes while maintaining a high emitter weight, and the long lamp life which goes with such a high weight. A further object is to reduce lamp end disclouration due to cathode disintegration.

A cathode structure according to the invention con- I sists of a number of fine tungsten wires braided to form a tubular braid which is filled with thermionically emissive material throughout its length with the exception of short end portions which are secured to the lead-in filled with thermionically emissive material extend for at least /2 mm. from the points at which the braid is secured to the lead-in wires.

While the tubular braid may extend in a straight line between the lead-in wires it is preferred to wind it into a helical form.

The thermionically-emissive material may be a mixture of barium, strontium, and calcium oxides.

By way of example one embodiment of the invention will now be described with reference to the accompanying drawing, in which:

FIG. 1 shows a low pressure fluorescent discharge tube incorporating a cathode structure in accordance with the invention, and

FIG. 2 is a general view of the cathode structure of the tube of FIG. 1.

FIG. 3 shows the cathode wound in double helix form.

3,250,943 Patented May 10, 1966 "ice The fluorescent tube shown in FIG. 1 is 5 ft. in length, has a lamp loading of 80 watts, and is suitable for use with starter switches or cathode heating transformers. The lamp current is in the range of 0.7 to 0.9 amp.

The lamp has at each end a cathode 10. This cathode is formed by braiding eight strands of 2.8 mgm./200 mm. tungsten wire on a mandrel of 0.4 mm. molybdenum wire at a pitch of 0.29 mm. This gives a gap between adjacent wires of about 0.04 mm. Cylindrical braiding is employed in which half the wires are wound around the mandrel or core in one direction and half in the opposite direction, each wire crossing alternately inside and outside those wound in the opposite direction.

The braided tube thus formed on the core of molybdenum wire is coiled on a secondary mandrel of 1.09 mm. diameter with a pitch of .8 8 mm. to form twelve complete turns. At each enda straight portion of 4 to 5 mm. is left. desired shape and the molybdenum core is then dissolved away.

As shown in FIG. 2 the completed cathode 10 is clamped to lead-in wires 11 and 12 extending from the lamp stem 13 by turning over the ends 14 and 15 of the lead-in wires 11 and 12 and gripping the ends of the filament under the turned-over ends of the lead-in wires.

The whole of the braided tube with the exeception of the end portions 16 and 17 to the left of the point X and to the right of the point X in FIG. 2 is now filled with emitter material consisting of a mixture of barium, strontium and calcium carbonates and processed to reduce this material to a mixture of oxides filling the whole of the braided tube between the points X and X The end portions 16 and 17 extend at least /2 mm. from the point at which they are secured to the lead-in wires 11 and 12 and are preferably of a length of 1 to 2 mm.

The cathode 10 is mounted in a lamp of conventional form having an envelope 20 and end caps 21 and 22 provided with terminal pins 23 and 24 respectively to which the lead-in wires are connected. Nickel strips 25 and 26 are mounted alongside the cathode to reduce damage to the cathode resulting from electron bombardment.

In an alternative construction shown in FIG. 3 the braided tube, instead of being wound into a simple helix between the lead-in wires, is wound in a double helix 27 whose axis is parallel to that of the cylindrical lamp envelope. The arrangement is otherwise the same as in I FIG. 2 and the same reference numerals have been used.

The resistance of such a cathode construction should be between 2.5 and 2.7 ohms. at 20 C. Lamps with such cathodes will start on a transformer heating circuit within 0.3 second, compared with 1.0 to 1.5 seconds for coiled coil cathodes made from 35 mgm./200 mm. tungsten wire. The ratio of emitter weight to wire weight in the case of the braided cathode is 17 mgm./28 mgm.=0.6

compared with 11 mgm./ mgrn.=0.l5 for the coiled coil. The estimated life for lamps with a braided cathode structure is 2.5 times the life of lamps with coiled coil cathodes operating under similar conditions.

The fact that the braided tube is not coated with emitter material at its ends facilitates reduction of the emitter material to oxides by passage of current through the cathode during the manufacture of the lamp since none of the emitter material is in thermal contact with the lead-in wires and the necessary high temperature can therefore be easily attained over the whole length of the filled portion. The unfilled end portions also have a smaller thermal inertia than the remainder of the cathode and the rapid heating of these portions and the formation of hot spots adjacent the ends of the filled portion contributes to rapid starting of the lamp.

The coiled tube is heat treated to set it to the A braided cathode will operate satisfactorily over a larger current range than a coiled coil cathode because a hot spot can be maintained on a single fine wire of the braid at a current which would be too low to maintain the emission temperature 011 a heavier Wire forming the coiled coil.

We claim:

1. A cathode structure consisting of a number of fine tungsten wires braided to form a tubular braid which is filled with thermionically emissive material throughout its length with the exception of short tubular end portions and means for securing said tubular end portions to the lead-in wires.

2. A cathode structure as claimed in claim 1 in which the braid is clamped in the lead-in wires.

3. A cathode structure as claimed in claim 2 in which the end portions not filled with themionically emissive material extend for'at least /2 mm. from the points at which thebraid is secured to the lead-in wires.

4. A cathode structure as claimed in claim 3 in which the braid is wound into a helical coil.

5. A cathode structure as claimed in claim 4 in which the helical coil is a double helix.

6. A cathode structure as claimed in claim 5 in which the thermionically emissive material is a mixture of barium, strontium and calcium oxides.

7. A discharge tube having a cathode structure as claimed in claim 3.

8. A low-pressure fluorescent discharge tube having a cathode structure as claimed in claim 5.

References Cited by the Examiner UNITED STATES PATENTS DAVID J. GALVIN, Primary Examiner.

BENNETT G. MILLER, Examiner. 

1. A CATHODE STRUCTURE CONSISTING OF A NUMBER OF FINE TUNGSTEN WIRES TO FORM A TUBULAR BRAID WHICH IS FILLED WITH THERMIONICALLY EMISSIVE MATERIAL THROUGHOUT ITS LENGTH WITH THE EXCEPTION OF SHORT TUBULAR END PORTIONS AND MEANS FOR SECURING SAID TUBULAR END PORTIONS TO THE LEAD-IN WIRES. 